8.2.2 Upper respiratory disease
A large, clear airway is an essential requirement for athletic function. Static or dynamic obstructions of the upper airway are a common cause of poor athletic performance. Static obstructions are easily observed endoscopically at rest. To visualise a dynamic obstruction, endoscopy is best performed with the horse exercising on a treadmill.
Upper airway obstructions result in abnormal respiratory sounds at exercise. It is therefore important that the rider is carefully questioned about respiratory noise even in a horse which is presented for poor athletic performance which is susto be due to a cardiac problem. The way in which animals perform during a race can also be important in reaching a diagnosis. Most horses with heart disease start the race well and then fade. Occasionally they will stop suddenly, but this history should also suggest the possibility of a dorsal displacement of the soft palate (DDSP).
Clinical examination of the poor performance horse should include careful palpation of the larynx. Evidence of atrophy of the dorsal crico-arytenoideus muscle is suggestive of a significant degree of recurrent laryngeal neuropathy (RLN). Exercise may be required to reproduce an abnormal respiratory noise which has been noted by the owners, for the benefit of the veterinarian.
A roaring or whistling sound is characteristic 6f RLN. A 'gurgling' noise is characteristic of DDSP, colloquially described as 'choking-up'. Occasionally, other forms of obstruction such as epiglottal entrapment, laryngeal chondritis, fourth brachial arch defects, sub-epiglottic cysts and abnormalities of the alar folds can occur and may produce an abnormal sound.
The most important aid to the diagnosis of upper airway disease is endoscopy. Endoscopy in the resting horse is usually sufficient for a diagnosis to be made in the majority of cases. Animals with dynamic obstructions require endoscopy on a treadmill or a presumptive diagnosis from the history, clinical signs and description of upper respiratory noise from the jockey. Laryngeal hemiplegia resulting from RLN, nasal chamber obstruction, epiglottic entrapment, laryngeal chondritis and pharyngeal masses such as sub-epiglottic cysts can usually be observed by resting endoscopy. DDSP is often seen during resting endoscopy, but may not be a significant finding unless it is persistent. It is best demonstrated during exercise. If it is found, careful consideration should be given to underlying predisposing factors. Collapse of the pharynx is sometimes observed during exercise. Intermittent epiglottic entrapment has also been observed during treadmill endoscopy.